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Home , Calcium nitrate, Calcium sulfate

Gypsum and Other Sources Their properties and chemical processes when applied to soil. DR. JIM WALWORTH

alcium can help stabilize the aggregate structure of soils. The Cmost commonly used calcium sources include , agricultural lime, and a few other calcium salts. In some soils, existing calcium minerals can be dissolved, releasing the calcium they contain. It is important to be familiar with the properties of these various calcium materials and to understand the chemical processes that occur when additives are applied to soil.

IS CALCIUM EFFECTIVE? In this brief article there is not space to provide a thorough review of the interactions between cations and soil particles. In brief, negatively charged soil clay particles can be bound together into clumps or aggregates by positively charged molecules (cations). The formation of stable soil aggregates, a process called flocculation, encour­ Golf course soils may become dispersed when sodium (Na+) accumulates in ages infiltration and drainage, relation to calcium and magnesium. Dispersed soils have very poor water and prevents surface soil crusting. infiltration properties and may ultimately result in turf loss. Calcium can help Flocculation is promoted by high levels stabilize the aggregate structure of such soils. of salinity (which may not be conducive to plant growth) and by the presence Calcium is clearly the cation of choice CALCIUM ALREADY PRESENT of cations that are strong flocculators. for flocculating soil clays. Let’s look at the first option, solubilizing The dominant soil cations in medium- In soils without adequate soluble existing soil calcium. This strategy to high-pH soils are the monovalent Ca2+, increasing the Ca2+ in solution works only if there is an excess of cations (one positive charge per mole- will help to flocculate clay particles. minerals in the cule) sodium (Na+) and potassium (K+), There are two methods that can be soil. Soils with excess or solid-phase and the divalent cations (two charges used to accomplish this process. One calcium carbonate (CaCO3) are referred per molecule) magnesium (Mg2+) and is to solubilize calcium already present to as calcareous soils. They can be calcium (Ca2+). In highly acidic soils the in the soil; the other is to add a identified through a soil analysis. Look trivalent aluminum cation (Al+3) may be supplemental calcium source. for free lime on the soil test. It will present. The ability of the dominant soil cations to flocculate soil clays is a Table 1 function of their charge and size, and Relative flocculating power of major soil cations is shown in Table 1. In this table the Chemical Symbol Relative Flocculating Power flocculating power of Na+ is assigned a value of 1, and the other cations are Sodium Na+ 1.0 + assigned values relative to Na . We Potassium K+ 1.7 can see that K+ is a stronger flocculator than Na+, but that Mg2+ and Ca2+ are Magnesium Mg2+ 27.0 much more powerful flocculators than Calcium Ca2+ 43.0 either of the monovalent cations.

Green Section Record Vol. 50 (11) TGIF Record Number 204766 May 25, 2012 Page 1 , such as calcium

(CaSO4). Chemically, these two salts are closely related, the difference being that anhydrite does not contain water. Con- sequently, calcium sulfate anhydrite contains more calcium on a weight basis than gypsum. Calcium sulfate anhydrite contains 29.4% calcium, whereas gypsum contains 23.2% calcium. Both of these calcium salts are mined and then ground into a powder for use as soil additives. Additionally, by-product gypsum materials and waste products of phosphate production () and from power plant stack scrubbers (flue gas desulfurization gypsum) also are used. Sulfuric or sulfurous acid applied directly to a calcareous soil creates a reaction Gypsum is a good choice for calcium whereby lime is dissolved, producing calcium sulfate, carbon dioxide (seen as the addition because it is inexpensive, non- fizzing bubbles), and water. toxic, safe to handle, and it is relatively soluble. We are interested both in usually be reported in general cate­ requires several weeks to months to (how much of the will dis- gories such as high, medium, or low. take place (depending on soil condi­ solve in the soil water) and the rate of You can test for the presence of tions), unlike acids, which react dissolution (how fast the salt dissolves carbonates yourself by putting a drop instantly. in water). Mined gypsum is well-crystal- of dilute acid on them and observing Acids and acid-forming materials will lized, having formed over millions of whether or not they effervesce (fizz) only be effective in calcareous soils. years. Waste gypsum, on the other as the CaCO3 reacts with the acid The soil should effervesce when acid hand, is formed rapidly during industrial ( in the equation below) to is applied, or have medium to high or processes and is less crystallized. produce carbon dioxide (CO2) gas: very high free lime soil test levels. Although they have the same , the waste gypsum materials H SO + CaCO ➝ CO + H O + Ca2+ + SO 2- 2 4 3 2 2 4 BEST CALCIUM ADDITIVE dissolve more rapidly than mined In calcareous soils, acid can Now let’s look at calcium additives. gypsum. Sometimes powdered gypsum be applied to dissolve soil calcium There are several calcium-bearing is prilled in order to reduce dust and to carbonate. The products of the salts that can be used to add Ca2+ to improve handling properties, and this reaction of calcium carbonate and soil, but in order to be effective they slows its rate of dissolution. A study sulfuric acid are CO2, water (H2O), must be soluble. A salt is a compound that compared dissolution rates of 2- 2+ 2+ sulfate (SO4 ), and Ca . The Ca made up of a cation and an anion (a gypsum sources found that flue gas released from the soil CaCO3 can negatively charged molecule). Calcium gypsum dissolved 3.6 times faster now act as a flocculant. salts, of course, contain Ca2+ as their than mined gypsum, whereas phos- Any acid can dissolve soil calcium cation. The anion could be sulfate phogypsum dissolved 2.2 times faster 2- carbonate and release the bound (SO4 ) for calcium sulfate, carbonate than mined gypsum (Bolan et al., 1991). 2- calcium. Sulfuric acid is most common (CO3 ) for calcium carbonate, chloride The rate of dissolution is particularly - - because it is relatively inexpensive (Cl ) for , (NO3 ) important for treatment of soil crusting, and adds less salt to the soil than for , etc. which is caused by dispersion of clay hydrochloric acid (HCl). Sulfurous acid particles at the soil surface. In this

(H2SO3) can be produced by com­ GYPSUM AND situation, rapid dissolution is critical to bustion of elemental in a sulfur CALCIUM SULFATE ANHYDRITE maintain a high level of dissolved Ca2+ burner, and it is a popular alternative to The most widely used calcium soil in the surface soil as raindrops or sulfuric acid. Additionally, acid-forming additive is gypsum. Gypsum is one irrigation water leach cations from the materials such as elemental sulfur can of the family of calcium sulfates. The uppermost layer of soil. However, for be used. Elemental sulfur is converted chemical formula for gypsum is general treatment of soil structure, the . to sulfuric acid by sulfur oxidizing CaSO4 2H2O. This means that each rate of dissolution is less important bacteria, producing the same effect as gypsum molecule contains one calcium than the overall solubility. sulfuric acid. Sulfur conversion is a cation, one sulfate anion, and two Calcium sulfate anhydrite also can biological process, however, and . There are other calcium be used as a calcium supplement. The

Green Section Record Vol. 50 (11) May 25, 2012 Page 2 although agricultural lime may be a plant nutrient. It is needed for cell combination of calcium and magnesium division and growth and for redistribu­ carbonates if it is made from dolomitic tion of carbohydrates within the plant. rather than calcitic deposits. Calcium deficiency related to lack of The main use of lime is to raise soil pH available soil calcium is rarely encoun­ (to reduce acidity). In the same manner tered in moderate- to high-pH soils

that CaCO3 neutralizes sulfuric acid in and is usually limited to very acidic or the equation above, it also neutralizes sandy soils. Calcium supply and trans- acidity in low-pH soils. Unlike gypsum location within plants is dependent on and calcium sulfate anhydrite, lime an adequate and continuous supply of solubility is dependent on soil pH. Its water. As such, calcium deficiencies solubility increases in acid soils and are usually the result of drought stress decreases as soil pH increases. When rather than low soil calcium levels. soil pH is above approximately 8.2, However, if the supply of available soil lime becomes quite insoluble. This is calcium is inadequate, supplemental why most soils with a pH above this calcium can improve plant nutrition. In threshold are also calcareous, meaning this case, any soluble calcium material that they contain solid mineral calcium can be used to alleviate calcium carbonate. In acidic soils, supplemental deficiency. Lime (in acidic soils only) calcium carbonate will dissolve, but in and gypsum are the most widely used alkaline soils it will not; adding calcium soil-applied calcium , whereas carbonate to calcareous soils calcium chloride and calcium nitrate accomplishes nothing. are often used for foliar application.

CALCIUM CHLORIDE SUMMARY AND CALCIUM NITRATE Acid or acid-forming amendments are Calcium salts that contain Ca2+ and a acceptable additives for increasing monovalent anion such as Cl- (calcium soluble Ca2+ in calcareous soils only. chloride) or NO - (calcium nitrate) are In all soils, regardless of pH, gypsum is The pH of calcareous soils is typically 3 very highly soluble. They are not a good Ca2+ additive. Calcium sulfate greater than 8.0 and they often contain 2+ generous deposits of calcium carbonate usually used as calcium amendments anhydrite will also supply Ca , but it (lime). Much of the calcium in the lime because of their expense and their will dissolve more slowly than gypsum. is insoluble, especially when the pH high salt content. Applying enough of With either gypsum or calcium sulfate value is greater than 8.2. Acid and these salts to promote soil aggregation anhydrite, it is important to know the acid-forming materials can increase would generally increase soil salinity to composition of the material you select. the available calcium by chemically unacceptable levels. reacting with the lime. DR. JIM WALWORTH, walworth@ CALCIUM FOR PLANT USE ag.arizona.edu, Department of Soil, Calcium is a critical component of Water and Environmental Science of gypsum and calcium cell walls and is therefore an essential University of Arizona. sulfate anhydrite are similar, however the dissolution rates differ. Published reports indicate that the dissolution rate Table 2 of calcium sulfate anhydrite is slower Calcium additives and their appropriate use as affected by soil pH than that of gypsum – anywhere from Soil pH 5% to 72% that of mined gypsum. In Material <7.0 >7.0 Comments addition to the chemical composition, the dissolution rates of both gypsum Acids NO YES* Only effective in calcareous soils (*calcareous and calcium sulfate anhydrite are soils contain lime and usually have a pH>8.0) dependent on type and degree of Gypsum YES YES Will not change soil pH crystallization, particle size, presence of impurities, and method of manu­ Lime YES NO Raises soil pH; not soluble in higher-pH soils facture for non-mined salts. CaSO4 YES YES Dissolves more slowly than gypsum; will not (Anhydrite) prevent surface crusting LIME Calcium carbonate or limestone is CaCl2 YES YES Can raise soil salinity to unacceptable levels another mined calcium salt. It’s often Ca(NO3)2 YES YES Can raise soil salinity to unacceptable levels referred to as lime or agricultural lime,

Green Section Record Vol. 50 (11) May 25, 2012 Page 3